1. Field of the Invention
The invention relates to an improved intra-aortic balloon catheter. More particularly, the invention relates to an intra-aortic balloon catheter having an ultra-thin stretch blow molded balloon membrane with improved abrasion resistance, fatigue life, and aneurization resistance.
2. Description of the Prior Art
Intra-aortic balloon (IAB) catheters are used in patients with left heart failure to augment the pumping action of the heart. The catheters, approximately 1 meter long, have an inflatable and deflatable balloon at the distal end. The catheter is typically inserted into the femoral artery and moved up the descending thoracic aorta until the distal tip of the balloon is positioned just below or distal to the left subclavian artery. The proximal end of the catheter remains outside of the patient's body. A passageway for inflating and deflating the balloon extends through the catheter and is connected at its proximal end to an external pump. The patient's central aortic pressure is used to time the balloon and the patient's ECG may be used to trigger balloon inflation in synchronous counterpulsation to the patient's heart beat.
Intra-aortic balloon therapy increases coronary artery perfusion, decreases the workload of the left ventricle, and allows healing of the injured myocardium. Ideally, the balloon should be inflating immediately after the aortic valve closes and deflating just prior to the onset of systole. When properly coordinated, the inflation of the balloon raises the patient's diastolic pressure, increasing the oxygen supply to the myocardium; and balloon deflation just prior to the onset of systole lowers the patient's diastolic pressure, reducing myocardial oxygen demand.
Intra-aortic balloon catheters may also have a central passageway or lumen which can be used to measure aortic pressure. In this dual lumen construction, the central lumen may also be used to accommodate a guide wire to facilitate placement of the catheter and to infuse fluids, or to do blood sampling.
Typical dual lumen intra-aortic balloon catheters have an outer, flexible, plastic tube, which serves as the inflating and deflating gas passageway, and a central tube therethrough formed of plastic tubing, stainless steel tubing, or wire coil embedded in plastic tubing. A polyurethane compound is used to form the balloon.
All IAB catheters have two opposing design considerations. On the one hand, it is desirable to make the outer diameter of the entire catheter as small as possible: to facilitate insertion of the catheter into the aorta, maximizing blood flow past the inserted catheter, and to allow for the use of a smaller sheath to further maximize distal flow. On the other hand, however, it is desirable to make the inner diameter of the outer tube as large as possible because a large gas path area is required to accomplish the rapid inflation and deflation of the balloon. As a result of these opposing design considerations there is a need for a smaller catheter with a larger gas path area.
One method of making the outer diameter of the wrapped balloon portion of the catheter as small as possible is to wrap the balloon in its deflated state as tightly as possible around the inner tube. Wrapping the balloon tightly, however, has posed a number of difficulties. First, it is difficult to wrap the balloon tightly because of the friction between contacting surfaces of the balloon. Second, contacting surfaces of a tightly wrapped balloon may stick upon initial inflation. Datascope Investment Corp.'s co-pending application Ser. No. 08/958,004, filed on Oct. 27, 1997, herein incorporated by reference in its entirety, discloses a lubricous coating for the balloon membrane which solves the above mentioned problems. The coating allows the balloon membrane to be wrapped tightly more easily and prevents sticking of the balloon membrane upon initial inflation.
A second method of making the outer diameter of the wrapped balloon portion of the catheter as small as possible is to decrease the size of the inner tube. Datascope Investment Corp.'s co-pending application Ser. No. 08/958,004 also discloses an intra-aortic balloon catheter with an inner tube having a smaller diameter only in the portion enveloped by the balloon membrane.
Although the above two methods have substantially reduced the overall insertion size of the intra-aortic balloon catheter the need still exists for greater size reduction. Furthermore, the need also exists for a balloon membrane with improved abrasion resistance, fatigue life, and aneurization resistance. Currently, the method of manufacturing polyurethane balloon membranes is solvent casting. This casting method does not provide the formed membrane with ideal physical and mechanical properties. A solvent caste membrane with the basic mechanical properties necessary for balloon pumping, typically has a single wall thickness of 4 to 6 mils (a mil is equal to one thousandth of an inch) which leads to a relatively large wrapped diameter of the balloon membrane. A thin solvent caste polyurethane membrane is capable of being manufactured, however, such a membrane does not demonstrate the required abrasion resistance and fatigue life. Therefore, the need exists for an improved method of making a balloon membrane which will allow for a balloon membrane having a reduced thickness, and at the same, having improved mechanical properties, including an improved abrasion resistance, fatigue life, and aneurization resistance.
The present invention comprises an intra-aortic balloon catheter having a stretch blow molded balloon membrane. The balloon membrane is made from thermoplastic elastomeric and/or semicrystalline materials such as but not limited to polyurethane and polyetheramid. As discussed above, intra-aortic balloon membranes are generally solvent cast.
The process of stretch blow molding catheter balloon membranes is known in the art. However, intra-aortic balloons have been traditionally made by solvent casting because intra-aortic balloon membranes require special characteristics: they must be substantially nondistensible and have high abrasion resistance, fatigue life, and aneurization resistance. Stretch blow molding has been traditionally used for angioplasty balloon membranes. These balloons are generally made from PET, Nylon, or PEBAX materials. These materials achieve their high strength at least partially because of the crystallization formed in their microstructure during the initial stretching step of the tube and as a result of quickly cooling the tube to a temperature below the crystallization temperature of the tube material. Crystallization of the microstructure increases the strength of the balloon membrane, however, as the inventors of the present invention have discovered, it has a negative effect on the abrasion resistance and fatigue life of the balloon membrane. Given that angioplasty balloon/PTCA therapy is a short duration therapy, crystallization is generally not a problem. Actually, it is quite useful given that it enhances the strength of the balloon material. Intra-aortic balloon therapy, on the other hand, involves repetitive inflation and deflation of the balloon membrane over a longer period of time. Accordingly, it is known in the art that stretch blow molded balloons are not appropriate for intra-aortic balloon membranes, which require high strength as well as high abrasion resistance and fatigue life.
The present invention overcomes the above described obstacle by relying on the increased strength of polyurethane resulting from the high orientation and molecular interaction of the polyurethane molecules along the longitudinal axis of the tube. Said orientation results from stretching the tube until substantially all stretchability is removed. Polyurethane, and the other materials listed in the present application, do not exhibit significant stress induced crystallization upon stretching. Accordingly, the inventors of the present invention have discovered a means to create a balloon membrane strong enough to endure intra-aortic balloon pumping therapy without creating crystallization microstructure, which they have discovered, is detrimental to the abrasion resistance and fatigue life of the balloon membrane.
U.S. Pat. No. 5,370,618 to Leonhardt discloses a pulmonary artery balloon catheter comprising a catheter terminating in a blow molded polyurethane balloon membrane. Pulmonary artery catheters are generally used for blood pressure measurements. Upon insertion and placement of the catheter the balloon membrane is inflated, occluding the housing blood vessel, so as to create a measurable pressure differential on either side of the balloon membrane. In order to achieve complete occlusion of the housing blood vessel the pulmonary artery catheter balloon membrane is elastic so as to allow expansion of the membrane. This is in contrast to the balloon membrane of the present invention which is stretch blow molded and is specifically manufactured to substantially eliminate distensibility in the final product.